Low flying aircraft, especially helicopters, run the risk of colliding with electrical power transmission lines. These lines which are suspended from pylons, are not easy to see, and present a considerable hazard to such aircraft. One approach for detecting the proximity of a source of an alternating magnetic field, such as that emanating from a power line, is disclosed in the U.S. Pat. No. 4,362,992 of Young, et al. As disclosed therein, the system includes a sensor coil which is sensitive to the magnetic field. Processing means are responsive to the signal induced in the coil and produce a warning signal which is indicative of the proximity of the detected magnetic field. The system also includes cancelling means for cancelling the earth's field in the region of the sensor coil. As disclosed in the patent, the sensor coil may be one of a pair which are operable to provide a signal which is representative of the directional heading of the magnetic field.
A more recent approach for warning an aircraft pilot of a potential impact with a power line is disclosed in the United States Patent of Merritt et al., U.S. Pat. No. 5,252,912. The Merritt et al. system includes an antenna for sensing the electric field associated with a power line and producing an electric field signal. The system also includes means for generating a time-to-impact signal which is representative of the time for the aircraft to reach the power line if it were to continue on its path of movement. In this system, sensors are used to determine the direction of the power lines.
It is now believed that there may be a relatively large commercial market for an airborne power line detector and warning system in accordance with the present invention. There should be a relatively large demand because such systems are highly reliable and yet relatively inexpensive to manufacture and install. The systems in accordance with the present invention are of minimal weight and size, easy to install and service, durable, and at the same time provide a clear warning to a pilot that the aircraft is flying in a vicinity which is occupied by power lines. The system in accordance with a preferred embodiment of the invention also includes a built in back up system which operates independently of the first system and which is operable even though the power lines have been shut down. In addition to the above, it is believed that the power line detector and warning systems disclosed herein will overcome to a large degree the shortcomings of the prior devices, as will become apparent from a reading of the detailed description of the invention.